The invention relates to a low-pressure sodium discharge lamp provided with
an evacuated, tubular glass outer bulb which is closed in a gastight manner PA1 with a first end portion where a glass stemtube enters the outer bulb, having a pinch seal inside the outer bulb, which end portion supports a lamp cap provided with contacts, and PA1 with a second end portion having a dome shape; PA1 a glass discharge tube bent into a U-shape with end portions which are each closed in a gastight manner and each have a pinch through which a respective current conductor is passed to an electrode arranged in the end portion in question, which discharge tube is filled with sodium and rare gas; PA1 electrical conductors which extend each from a respective contact of the lamp cap through the pinch seal so as to be connected to a respective current conductor; PA1 a substantially plane mica plate with openings through each of which a respective pinch is passed, acting as a first centring member which keeps the discharge tube centred in the outer bulb, and a second centring member for the discharge tube in the second end portion of the outer bulb.
Such a low-pressure sodium discharge lamp is known from GB 865 928-B.
During manufacture of this lamp, the outer bulb must be held with its first end portion upwards while the outer bulb is being fused to the stemtube in order to prevent the discharge tube from dropping from the outer bulb. This is a disadvantage because a quicker and more reproducible fusion is obtained when an outer bulb has an excess longitudinal dimension and is fused to the stemtube while its second end portion is pointing upwards. When the outer bulb is heated locally, it constricts in this location and fuses with the stemtube. The excess length portion then drops off because it loses its connection to the outer bulb. This fusion method is called "drop seal" for that reason.
The known lamp could be manufactured by the drop seal method if the second centring member, a resilient bracket in the known lamp, had a strong clamping action both around the discharge tube and in the outer bulb, as a result of which the discharge tube would be suspended. Such a centring member, however, would lead to a considerable price increase.
Another possibility would be to use a metal clamp near the first centring member. Such a metal clamp, however, would have the disadvantage not only of increasing the cost price of the lamp, but also that it may cause damage to the outer bulb. An evaporated getter is often present for maintaining a vacuum in the outer bulb, for example a barium mirror on the outer bulb wall. This mirror is obtained in that barium is evaporated from a holder after the outer bulb has been sealed. The holder is heated inductively for this purpose. A metal body, for .example a blade spring, in contact with the outer bulb becomes red-hot then and strongly heats the outer bulb locally. This may cause stresses leading to fractures.